In 2004, the animals were purchased by Downtown Aquarium in Houston, Texas and since then have been kept indoors, behind glass, in a decorative tank.

Campaigners have been fighting to get the animals moved to a more suitable habitat - one with natural sunlight.

The tigers have no access to sunshineCredit:
Facebook/Downtown Aquarium Houston

The indoor exhibit has the appearance of a movie set, and no access to natural light.

Their only access to the outside world is a piece of glass, through which visitors to the aquarium gawk at them.

Nero, Marina, Coral and Reef are all aged between 12 and 13, and live in the aquarium's "Maharaja's Temple" attraction.

On the top floor of the building is a restaurant, and below it is an aquarium, featuring the tigers.

When they aren't on display, the tigers are kept in bare metal cages in back rooms of the facility.

When not on display, the tigers are kept in cagesCredit:
ANIMAL LEGAL DEFENSE FUND

Dr Jennifer Conrad, a vet who works with the Animal Legal Defense Fund (ALDF), said in a statement: "The dungeon-like conditions that the tigers are forced to endure at Houston's Downtown Aquarium harm their physical health and psychological well being, and deny them much that is natural and important to a tiger".

"It is cruel to confine complex, roaming carnivores such as tigers to a tiny, dark, artificial, unenriched enclosure where they never see any daylight, much less bask in sunshine.

"[They] are at risk for serious long-term, debilitating injuries from being forced to live on slippery, unyielding concrete their entire lives."

The ADLF is attempting to move the tigers to a more appropriate environment and on Monday served Landry's Inc, which owns the aquarium, with a notice of intent to sue over alleged violations of the Endangered Species Act.

Their notice of their intent to sue reads: "Indeed, the tigers have been observed pacing and lunging at the glass wall separating the tigers from the public — all common stress responses for tigers living in close confinement with no ability to seek privacy from the viewing public".

They are offering to rehome the tigers free of charge - but if the tigers are not moved to a reputable, accredited sanctuary, the ADLF will follow through with the lawsuit.

James Prappas, director of animal husbandry at the aquarium, has refuted the claims that the tigers haven't felt natural sunlight, telling The Dodo that there are some skylights in the exhibit which let light through.

He also said there is an appropriate air system which meets humidity and ventilation guidelines.

The ALDF is arguing that the tigers' current living conditions violate a section of a tiger care manual issued by the company that accredits the aquarium.

This manual states all tiger enclosures should have an outdoor space with natural vegetation and opportunities for the tigers to take part in natural behavior, for example running, jumping, climbing and being able to hide from the public.

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Landry's has threatened to counter-sue for libel, according to the Houston Chronicle, and has said the allegations made are "false and manipulative".

Steven Scheinthal, general counsel for Landry's, said: "Our tigers receive the highest level of care and treatment and have always exhibited the signs of well-maintained animals.

"We are aware of the proposed changes to the accreditation standards and once enacted, we will make every effort to comply to the new standards. If we are unable to make such changes, we will move our tigers to a new home."